The hydraulic motor may be of the rotating casing or rotating shaft type. It has two directions of rotation, i.e., depending on the principal port which is chosen for supply and that which is chosen for exhaust, the rotor may rotate with respect to the stator in the two opposite directions of rotation. Furthermore, the motor is reversible and, in one of its directions of rotation, it is capable of functioning as a pump, for example when the vehicle is descending a slope. It may be question of a slow motor (low speed and high torque) or of a fast motor. The motor used in such a circuit is intended to ensure drive of the members (for example tracks) for displacing machines such as hydraulic shovels in tmnslation.
The means for circulating hydraulic fluid conventionally comprise one or more pumps. The circuit may be an open circuit in which a pump with one sole outlet is used for supplying the principal conduit which serves for supply, while the conduit which serves for exhaust is connected to a reservoir without pressure. It may also be question of a closed circuit, in which a pump is used, having a high-pressure fluid outlet to supply the supply conduit and a low-pressure fluid inlet connected to the exhaust conduit.
The pump may present fixed or variable capacity. It may have two directions of rotation in order selectively to supply one or the other of the principal conduits. It may also have one sole direction of rotation, in which case a selector of the connection of one or the other of the principal conduits to the outlet of the pump is used. In addition, the circuit may comprise a booster pump which, when the motor is stopped, makes it possible to maintain a so-called booster pressure in the whole of the conduits of the circuit and, when the motor is operating, to maintain the conduits connected to the exhaust at booster pressure.
In the rest position of the principal selector of the device for selecting the circulation of the fluid between the principal ports of the motor and the principal conduits to which they are respectively connected, the circulation of the fluid is prevented in the sense of exhaust from the two principal ports of the motor, so that the latter cannot operate. The principal selector is automatically placed in its rest position when the fluid pressures in the principal conduits are substantially equal. On the other hand, when the principal selector occupies one of its two active positions, the motor may operate in one or the other of its two directions of rotation.
The device for immobilizing the rotor of the motor is generally used as parking brake, which serves to maintain the motor stopped when the supply and exhaust in the principal conduits cease. In particular, at stop, the pressures in the principal conduits are substantially equal and the principal selector occupies its rest position, while the immobilization member occupies its active position. Of course, this immobilization member must occupy its inactive position for the motor to function under good conditions.
With certain earlier motors, it was provided to actuate the immobilization device separately to place the immobilization member in its inactive position and to set the motor in action, respectively generating a supply and an exhaust of fluid in each of the two principal conduits.
In order to avoid poor manipulations, it was then thought to render the brake de-activation control automatic. In this way, in certain circuits, the immobilization member is automatically urged from its active position to its inactive position when the circulation of fluid supplying one of the principal conduits and allowing exhaust in the other conduit, begins. Applicants have observed that this system was not entirely satisfactory. In effect, with such a system, the immobilization member is urged towards its inactive position at the same time as the supply conduit of the motor is placed under pressure and the exhaust conduit is connected to exhaust, with the result that the rotor already exerts a torque. The fact of inactivating the immobilization member under these conditions may constitute a cause of premature wear of certain elements of the motor, in particular of this immobilization member. Furthermore, this results in a risk of unpleasant jerks when the motor is started up.
It is an object of the present invention to overcome these drawbacks by proposing a circuit which, while allowing an automatic passage of the immobilization member into its inactive position when the motor is started up, causes the principal ports of the motor to be effectively connected to the supply and exhaust conduits only after the immobilization member has been urged towards its inactive position.